This invention relates to new methylene-bis(cyclohexylisocyanates) monosubstituted on only one cyclohexane ring and to a process for their production.
Aliphatic or cycloaliphatic diisocyanates, such as hexamethylene diisocyanate, 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethyl cyclohexane and 4,4'-methylene-bis-(cyclohexylisocyanate), and their position and/or stereo-isomer mixtures are used in polyurethane chemistry. Such diisocyanates are useful in the production of light-stable coating materials characterized by high resistance to weathering. In addition, isomer mixtures of such diisocyanates may be readily processed by reaction with polyols to form lacquer binders, elastomers, or foams provided that the diisocyanate is liquid at room temperature and compatible or adequately miscible with the polyols (See for example, German Offenlegungsschrift No. 1,768,832). Pure trans, trans-4,4'-methylene-bis-(cyclohexylisocyanate), for example, is not suitable for such processes because it is solid at room temperature (melting point 83.degree. C.) and is sparingly soluble in polyols. It is therefore removed from the reaction mixture by crystallization before the end of the polyaddition reaction.
The stereo-isomer mixture consisting of trans, trans-, cis, trans- and cis, cis-4,4'-methylene-bis(cyclohexylisocyanate) which is formed in the phosgenation of nucleus-hydrogenated 4,4'-diaminodiphenyl methane is also solid at room temperature. Because the trans, trans-diamine is obtained as the main product where hydrogenation is carried out under normal conditions, this stereoisomer mixture can be used only to a limited extent for producing polyurethanes.
Although the separation of trans, trans-4,4'-methylene-bis-(cyclohexylisocyanate) from the stereoisomer mixture is technically possible, for example by precipitating the trans, trans-isomer in the form of carbamic acid chloride, followed by filtration (See Japanese Patent Publication No. 53 046-944 of Apr. 27, 1978), the yield of isocyanate is regarded as a major disadvantage.
It is known that liquid cycloaliphatic diisocyanates can be obtained by the phosgenation of 2,4'-methylene-bis-(cyclohexylisocyanate) (See German Offenlegungsschrift No. 1,768,832). According to this publication, mixtures of 2,4'- and 4,4'-isomers are even liquid when the 2,4'-isomer content amounts to between 30 and 95 wt. % and the 4,4'-isomer has a trans, transisomer content of less than 50 wt. %. However, the 2,4'-isomer is difficult to obtain in pure form because it is obtained in 3 stages (by the condensation of aniline with formaldehyde, followed by nucleus hydrogenation and phosgenation) and 2,4'-diaminodiphenyl methane is obtained in a yield of only about 30% of the theoretical in the condensation stage, even under optimal conditions (See, German Offenlegungsschrift No. 1,937,685) Consequently, isolation of the pure 2,4'-isomer requires technically involved isomer separation.
The direct production of a methylene-bis(cyclohexylisocyanate) rich in 2,4'-isomers by phosgenating a corresponding methylene-bis-(cyclohexylamine)-isomer mixture is also disadvantageous because in the aniline/formaldehyde condensation stage, the 2,4'-diaminodiphenyl methane content correlates to that of the 2,2'-isomer. Further, the 2,2'-isomer undergoes decomposition during the nucleus hydrogenation stage.